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Shop Solutions: Smart Honing Delivers Tight Pump Tolerances

“We don’t believe it!” That’s how experienced machinists reacted to their first encounter with a machine tool that had produced nine bores per part with only 0.000020" (0.5 µm) variation, all day long, with essentially no operator attention on its first run of production parts.

That’s 2.21 Cpk process capability for a game-changing smart honing technology at Waltz Brothers Inc. (Wheeling, IL), a precision grinding and machining operation where flight-critical aerospace hydraulic pumps are produced by the hundreds every year.

Company president Larry Waltz knows it’s hard to hit this tolerance window honing one bore per part, as the shop had been doing with a standard machine. With nine opportunities to make a micron-sized mistake on an expensive workpiece, it takes great skill and many time-consuming machine/measure iterations to make these parts on a standard machine.

“The new SV-1015 honing system not only produces perfect bores, it also records its final air gaging measurements to track with each serialized part,” said Waltz. “When I showed this new machine to a customer for whom we’d been making these parts, they were so impressed they wanted us to commit to a blanket order for all their parts for the next five years,” Waltz said. “In that instant, we validated our painstaking decision to adopt this technology.”

“We’re a second-generation grinding business started by my father and uncles in 1939. Our focus is close-tolerance precision parts requiring many operations that typically conclude with grinding, honing, or lapping, so we added chip cutting to better control the processes upstream from grinding,” said Waltz. One specialty at this 60-person company is manufacturing parts that go into aerospace hydraulic actuation systems. “Piston pumps are a good niche for us,” Waltz said. “These are the most critical parts we make. There’s great variation in materials, design details, process flow and assembly requirements, but the parts do have fundamental similarities.”

The heart of the pump, called a cylinder block or rotor, starts as a turned blank up to 8" (203-mm) diameter. Then, nine circumferential piston bores from 0.1875 to 1.5" (4.8–38.1-mm) diameter are roughed-in on a machining center, and the part may be heat treated. The piston bores are not through holes, but have small kidney-shaped slots cut through the bottom. Some designs call for heat treating, use of bronze bore liners, or bronze plating on the bottom of the part, its running face. Liners may be cast in or produced on Waltz’s screw machines, then anchored into retaining grooves with a ballizing process or swaging.

“Nine bores are common to most all pump designs. We must maintain tolerances of 0.000020" [0.5 µm] for bore roundness, 0.000040" [1 µm] straightness, 4–8 µin. [0.1–0.25 µm] Ra bore finish, and less than 3 µin. [0.07 µm] surface on the pistons. We have three Zeiss CMMs that can scan the piston bores at various levels for cylindricity, too. All these parts are serialized, traceable to the material and processes used to create them,” said Waltz.

“During prototyping, we determine where to leave needed stock on the part, but we fine-tune the stock allowance over time and tweak our processes to be as efficient as possible. For example, heat-treated parts are often carburized which creates only a thin layer of surface hardness at about Rc 54. Thus, we want to remove as much metal as possible before hardening, so we aim for 0.0010 to 0.0015" [0.0254–0.0381-mm] honing stock in the bore in the hardened state. Parts with bronze bore liners are, of course, a different story.”Waltz had been using standard horizontal honing technology from Sunnen, where outcomes are heavily dependent on the operator’s skill. The operator would hone one bore, clean the part, air gage it at three different levels and then at 90° bottom/middle/top to see if there’s any issue, such as taper at the bottom of the bore which would have to be feathered out.

“There’s a tremendous amount of back-and-forth to complete nine bores, and if you blow one bore, you’ve scrapped a part valued at several hundred dollars. After honing, we must have confirming CMM inspections on each part, each bore, and this becomes part of the manufacturing history for each block. Honing and inspection could easily take two hours per part,” said Waltz.

“Our challenge to Sunnen was to automate everything, including part indexing, air gaging, and recording of gage readings for the part’s history. We achieved all this with the new Sunnen SV-1015, but it was one of the new patented standard features in this machine—multifeed honing—that has played a role in taking our honing process capability to a new level,” said Waltz.

The new patented multifeed honing capability gives users a choice of tool-feed modes to achieve the shortest cycle times, lowest part cost, and longest abrasive life. Multifeed combines Sunnen’s new controlled-force tool-feed with its existing controlled-rate feed system. The two different modes allow the user to select the better option to suit the workpiece geometry, material and tool type/size.

Typically, a production honing process is set up to use an abrasive tool with a combination of grit size and bond optimized for specific part conditions. Tool expansion to achieve the desired results and final size is programmed based on rate of time. However, when a batch of parts comes in with a different heat treatment, distortion or a size variation, the operator must intervene because the tool may expand too quickly and be damaged.

In the opposite case with a softer-than-normal or oversize workpiece, the tool will still expand at its programmed rate, when it might have been able to expand faster to reduce cycle time. Expansion at too slow a rate may also result in glazing of the honing stones, which won’t self-dress if the cutting force is too low. Typically, the operator tweaks a rate-feed system periodically to compensate for these variables.By servo-controlling the force in the tool feed system, however, the machine can sense and compensate for these variables. The controlled-force feature, which works in concert with the machine’s standard rate-feed system, functions like cruise control to maintain the optimum cutting load on the honing abrasive throughout a cycle, regardless of the incoming part’s hardness, geometry or size variation.

Waltz’s new SV-1015 vertical CNC honing machine has a single 5-hp (3.7-kW) AC spindle with 10-hp (7.5-kW) servo-stroking system and X-Y air bearing table. The operator loads the workpiece in a dedicated fixture that uses the part’s kidney slot as a locating feature. After each bore, the table indexes 90° to an Etamic air gaging station where the necessary readings are taken and stored. If the bore passes inspection, the table moves the part to a robot that indexes it for the next bore, duplicating this routine eight times to complete a part. A fixed-position master set ring inside the machine’s work envelope ensures correct gage calibration for current environmental conditions.

“On a recent project with an allowable bore tolerance of 0.000240" [0.006 mm], we easily held a tolerance range of 0.000060" [0.001524 mm]—that’s 25% of the total allowable tolerance, and we tripled the previous production rate, while reducing the labor component by 80% so the operator can do other work in the cell,” said Waltz. “We know from reinspecting the parts on our CMMs that the results correlate well. The process capability and data reporting features in the machine have been a great advantage to us and our customers, allowing that data to be downloaded to a spread sheet or SPC software. ME

For more information from Sunnen Products Co., go to www.sunnen.com, or phone 800-325-3670.

This article was first published in the December 2013 edition of Manufacturing Engineering magazine. Click here for PDF.